Many reactions of significance in biological systems, whether in living systems or in the laboratory, involve electrostatic interactions of charged species at a surface. For example, the use of cationic liposomes for the delivery of DNA into cells has been the subject of extensive study. The cationic carriers interact with the negatively charged DNA to form DNA-lipid complexes, which are believed to enter cells primarily by adsorptive endocytosis.
Fluorescently labeled probes have been used to study binding of DNA to cationic liposomes, as described, for example, in Zelphati et al., Biochim Biophys Acta 1390(2): 119–33 (1998). In this study, detection relied on a fluorescence quenching, which required that the molecule being detected (DNA) also be labeled, and that it be in close enough proximity to the probe to allow quenching. Zelphati also reported that background fluorescence from serum interfered with the measurements. Probes containing the pH-dependent fluorescent molecule 7-hydroxycoumarin have been used to study changes in electric potential at surfaces (Barenholz and Zuidam, Int J Pharm 183(1):43–6, June 1999). However, the use of such methods in studying lipid bilayers, especially in biological systems, is hampered by the instability of the probe in the bilayer.
Accordingly, there is a need for improved methods of observing such interactions, in a stable and reproducible manner.